Optically transmissive materials, such as glass or polymers may be used as a light guide to propagate light. A light guide typically includes at least one surface adapted to receive light from a light source and an optically smooth surface for reflecting light propagating through the light guide. Common examples of light guides include optical fibers traditionally used in the data communication industry and more recently light fibers used for illumination purposes. For example, U.S. Pat. No. 5,432,876 (the '876 patent) discloses one such illumination device employing light fibers. In this device, light may be injected into at least one end of a light fiber and allowed to exit the fiber at a predetermined position or positions along the length of the fiber. Light extraction structures or notches are formed in the core of the light fiber. The extraction structures define first and second reflecting surfaces, which reflect in a radial direction a portion of the light propagating axially through the fiber. The reflected light is directed at an angle that is less than the critical angle necessary for continued propagation along the fiber according to the principle of total internal reflection. As a result, the reflected light is extracted from the fiber. In contrast to prior techniques such as subjecting the fiber to relatively sharp bends, this system extracts light from the fiber in a controlled fashion.
Because the previously mentioned light extraction structures are formed from optically smooth surfaces, they reflect light by total internal reflection rather than by diffuse reflection. As a result, light is emitted from the light fiber in a pattern dictated by the configuration and arrangement of the extraction structures along the fiber. In general, when viewed at relatively close proximity, light from individual light extraction structures appears as discrete lines of light. That is, the light fiber generally will not appear to produce even illumination along its length, as does, for example, a fluorescent light. For example, when the extraction structures are sufficiently far apart from one another, the illumination pattern generated along the fiber is a series of bright stripes or lines of light alternating with regions of reduced intensity. In other cases, notably in planar light guides used to provide flat panel illumination, an image of the light source is transferred down the light guide so that the resulting illumination pattern includes an image of the light source, thus creating a nonuniform brightness level across the surface of the light guide.
A light fiber that appears uniform in brightness along its length is desirable for many applications. For example, such a light fiber could be used in place of neon lighting, which is often employed to form self-illuminated characters or letters or to outline the profile of a portion of a building.
U.S. Appl. Ser. No. 08/957,573 discloses a light fiber illumination device that employs roughened or uneven areas on the interior of the cladding, which serve as light extraction structures. The roughened areas comprise randomly dimensioned protrusions or pits that extract light in a diffuse manner. Because the extracted light is diffuse, it appears more uniform in brightness along the length of the fiber than the light extracted by extraction structures formed from optically smooth surfaces.
Unfortunately, because the roughened areas are not of optical quality but are rather random in size and orientation, it is difficult to control the amount of light radially extracted from the fiber and thus in general it is difficult to achieve high levels of brightness by this extraction technique. Thus, while such roughened areas facilitate the diffusion of light, they are less efficient at extracting the light than light extraction structures formed from optical quality surfaces.
To overcome the problem of reduced brightness, the previously mentioned patent application employs a diffuse reflective sheet of material that is disposed around a portion of the light fiber. The diffuse reflective sheet is used to reflect light back through the fiber at angles such that most of the reflected light passes through the fiber. Light that would otherwise be extracted and transmitted away from a viewer can then be observed, adding brightness to light extracted from an observable portion of the fiber. In essence then, this light fiber illumination device uses the diffuse reflective sheet primarily to increase the apparent amount of light that is extracted, while the roughened portions of the cladding are used to extract as well as diffuse the emitted light
However, since they are more efficient at extracting light, it would be advantageous to use optical quality light extraction structures rather than roughened portions of the cladding to extract light from the fiber. Unfortunately, a device using such an extraction technique typically appears to have discrete, non-uniform areas of brightness along its length.
The present invention is therefore directed to the problem of providing an illumination device that extracts light in a relatively efficient manner to enhance its brightness while producing even or uniform illumination along its length.